he sacroiliac joint is a true diarthrodial joint that joins the sacrum to the pelvis. The sacrum (tailbone) connects on the right and left sides of the ilia (pelvic bones) to form the sacroiliac joints. The pelvic girdle is made up of two innominate bones (the iliac bones) and the sacrum. The innominate bones join in the front of the pelvis to form the pubic symphysis, and at back of the sacrum to form the sacroiliac (SI) joints. Each innominate bone (ilium) joins the femur (thigh bone) to form the hip joint; thus the sacroiliac joint moves with walking and movement of the torso
in this joint, hyaline cartilage on the sacral side moves against fibrocartilage on the iliac side. The sacroiliac joint contains numerous ridges and depressions that function in stability. Studies have documented that motion does occur at the joint; therefore, slightly subluxed and even locked positions can occur.